This invention relates to a method of operating a compression ignition engine such as a diesel engine, and more particularly to a method of operating a compression ignition engine in which fuel is delivered to the or each combustion chamber of the engine by a mechanically operated fuel injection pump apparatus.
A mechanically operated fuel injection pump apparatus is typically driven by the engine, e.g. by the engine crankshaft, and includes one or more pumping chambers from which fuel is distributed for feeding directly into the or each combustion chamber by a or a respective injector, or indirectly, into a or a respective charge air airway or manifold, by an injector so that the fuel is mixed with the air in the combustion chamber or chambers. Fuel typically is injected into the or each combustion chambers directly or indirectly, prior to the piston in the combustion chamber reaching top dead centre (TDC), and typically the fuel will be ignited prior to the TDC piston position. Fuel may continue to be delivered to the combustion chamber as the piston moves away from TDC.
The fuel injector pump apparatus is operated to deliver to the or each combustion chamber a metered volume of fuel in accordance with demand set by an engine governor, and according to a timing regime i.e. at a selected crank angle before TDC when fuel injection commences and after TDC when fuel injection ceases, which volume of fuel and timing regime is generally arranged to ensure maximum fuel combustion and engine economy, and minimum emissions.
It is known that in normal use of the engine, when the engine operating temperature is raised to a normal operating temperature, a different fuel injection timing regime is desirable compared to when the engine is cold, i.e. on start up. Accordingly it is known to operate the fuel injection pump apparatus according to a different, advanced timing, regime during engine start up, to ensure fuel is provided in the or each combustion chamber earlier in the combustion cycle, i.e. earlier before combustion is initiated in the combustion chamber, which is earlier before the piston reaches TDC that is, at an advanced crank angle than would normally be the case.
In one kind of fuel injector pump apparatus, the timing regime is changed to a cold starting timing regime by operation of a solenoid by an engine controller, which controller responds to a sensed temperature, being the engine operating temperature or a temperature indicative of the engine operations temperature, when below a threshold temperature.
The solenoid may move a timing component of the fuel injection pump apparatus which advances the onset of fuel injection in each combustion cycle until the engine operating temperature reaches the threshold temperature, when the solenoid is de-energised and the timing component of the fuel injection pump apparatus moves back to a normal operating position so that the onset of fuel injection returns to a normal non-advanced regime.
It will be appreciated that when the fuel injection timing regime is advanced in response to the engine controller, when the engine operating temperature is below the threshold temperature, because fuel will have been provided in the combustion chamber earlier, when combustion commences in each combustion chamber cycle, there will be a surge in pressure in the combustion chamber.
As the torque developed by the engine increases, e.g. engine load increases, the maximum combustion chamber pressure in each operating cycle will commensurately increase.
For a particular engine which may have been designed according to certain engine parameters and/or operated according to design parameters, the design will have a maximum working combustion chamber design pressure which may be greater than any anticipated surge pressure experienced in the or a combustion chamber as a result of operating the engine according to an advanced timing regime.
However, commonly, basic engine designs are modified or their operation is modified, to permit the same basic engine design (i.e. using the same conversion hardware, i.e. engine i.e. the piston(s), rod(s), bearings, head gasket etc.) to be used for alternative applications. For example an engine may be modified or the engine's operation may be modified, in such a way that the maximum combustion chamber pressure is increased, closer towards the maximum design pressure. In such case, any surge in pressure as a result of the fuel injection timing being advanced, could result in maximum pressures in the or a combustion chamber which are not optimal for the engine design.
Moreover, such excess pressure will act on the conversion hardware of the engine (which may not be modified for the modified engine design or operation), and at least as the engine torque develops, the excess pressure could result in damage to or at least prematurely wear the conversion hardware of the engine.